Just try this...
http://sourceforge.net/projects/xonx/ (for XFree86 4.2.1.1)
You'll need to download and install in the order as they are listed below..
1. XInstall_10.1.sit
2. XFree86_4.2.0.1-10.1.zip and finally
3. XFree86_4.2.1.1.zip
and there you have XDarwin for your X.1.5 up...